As the customer reports at StackOverflow, sometimes marks individually hidden are still considered at CalcClickedPart.

The problem is actually at the TSeriesMarks.Clicked function, that doesn't consider the Items Visibility. So, if a Mark is ever drawn, its position and shape are defined and that's what is verified at that Clicked function, even if it has been hidden.

A simple example:

uses Series;

procedure TForm1.FormCreate(Sender: TObject);
var i: Integer;
begin
  Chart1.View3D:=false;
  Chart1.Legend.Hide;

  with Chart1.AddSeries(TLineSeries) as TLineSeries do
  begin
    for i:=0 to 8 do
    begin
      AddXY(i, 10);
      Marks.Item[i].Visible:=i<6;
    end;

    Marks.Show;
    Chart1.Draw;

    for i:=3 to Count-1 do
      Marks.Item[i].Hide;
  end;
end;


procedure TForm1.Chart1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
var ValueIndex: Integer;
begin
  Caption:='';

  ValueIndex:=Chart1[0].Marks.Clicked(X,Y);
  if ValueIndex>-1 then
  begin
    if Chart1[0].Marks[ValueIndex].Visible then
      Caption:='Mark'
    else
      Caption:='HIDDEN mark';

    Caption:=Caption + ' clicked at index ' + IntToStr(ValueIndex);
  end;
end;